The present invention relates to semiconductor laser diodes, particularly to AlGaAs-based laser diodes of high output power. Such laser diodes are commonly used in opto-electronics, often as so-called pump lasers for fiber amplifiers in the field of optical communication, e.g. for Erbium-doped fiber amplifiers. Specifically ridge waveguide laser diodes are suited to provide the desired narrow-bandwidth optical radiation with a stable light output power in a given frequency band. Naturally, output power and stability of such laser diodes are of crucial interest. The present invention relates to an improved laser diode, the improvement in particular concerning the structure and design of the ridge waveguide; it also relates to a manufacturing process for such laser diodes.
Semiconductor laser diodes of the type mentioned above have become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted, which improves speed as well as reliability within such systems.
In one kind of optical fiber communication systems, the laser diodes are used for pumping Erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance are ridge waveguide laser diodes with a power output of 150 mW or more, whose wavelengths match the Erbium absorption lines and thus achieve a low-noise amplification. Several laser diodes have been found to serve this purpose well and are used today in significant numbers. However, the invention is in no way limited to such laser diodes, but applicable to any ridge waveguide laser diode.
Generally, laser diode pump sources used in fiber amplifier applications are working in single transverse mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot lasers. Two main types are typically being used for Erbium amplifiers, corresponding to the absorption wavelengths of Erbium: InGaAsP at 1480 nm; strained quantum-well InGaAs/AlGaAs laser diodes at around 980 nm.
Semiconductor laser diodes of the types mentioned above have a number of problems. One particular significant one is that with increasing operating light output powers of vertically and laterally single mode semiconductor laser diodes, the maximum useable light output power is limited. It is believed that this is due to various reasons:
A limited linear power due to coherent coupling of the zero order mode with higher order modes.
A thermal rollover, i.e. a reduction of the light output power efficiency, due to ohmic heating. An excessive heating leads to a drastic increased carrier leakage over the laser diode hetero barrier. As a results of the light output power decreases with increasing temperature.
A catastrophical optical mirror damage, a so-called COD.
These limitations/damages seem to occur at certain power levels and are believed to be caused by increased thermal and opto-electronic influences on the waveguide like, e.g. spatial-hole burning and ohmic heating can results in a locally increased refractive index since n depends on the free carrier densitiy Ne,p and temperature T n=n(Ne, Np, T). In an edge emitting single mode waveguide laser diode, the optical intensity typically increases towards the front facet. Usually, the latter has a reduced reflectivity compared to the back facet, due to the mirror coating: The coating of the front facet has a reflectivity between 0.1% and 10%, while the back facet coating has between 70% and 100%.
Consequently, ways have been sought to prevent the above mentioned damages and overcome the limitations. One attempt to improve the light output power of semiconductor laser diodes is described in Lang et al U.S. Pat. No. 6,014,396. Lang et al disclose how to slightly broaden the ridge waveguide sections towards the front facet and the back facet to the same aperture. This reportedly reduces the series resistance compared to a standard narrow stripe device. In addition, the lateral gain regime is increased where the power density increases due to the asymmetric mirror coating of the front an back mirror. The effect of spatial hole burning is reportedly reduced when compared to a standard narrow stripe ridge waveguide.
However, the design proposed by Lang et al is not satisfactory from all points of view for ridge waveguide like laser diodes. Whereas Lang et al. propose to widen the waveguide to 20-50 xcexcm, standard single mode lasers with a width of more than 6-7 xcexcm have been found unstable concerning lateral single-mode operation within the operating regime, especially when optical feedback, i.e. with a Fiber Bragg Grating (FBG), is introduced. Also, Lang et al propose to use different facet cross sections or apertures. This requires a manufacturing process whereby the laser diodes are pair-wise placed symmetrically on a wafer. In other words, when the back section of the ridge waveguide is straight, while the front region is flared, the chip pattern for the manufacturing process is designed such that adjacent laser diodes are arranged face to face. This arrangement however leads to problems in chip handling, laser diode characteristics, and reliability and is thus rather cumbersome. Clearly, a process where all laser diodes are oriented in the same direction is by far preferable for the realisation of ridge waveguide like laser diodes.
Thus, it is a general object of this invention to devise a reliable design for a high power ridge waveguide which avoids the above-mentioned problems of high power laser diodes and which in particular provides a stable and high light output under all operating conditions and a sufficiently long life of such laser diodes.
It is a more specific object of this invention to provide a ridge waveguide laser diode design including at least one specifically tapered segment or flared region towards one of the waveguide""s ends, thus providing the desired stable high power output.
It is a further primary object of this invention to provide an advantageous and economical manufacturing method for the novel ridge waveguide laser diodes, allowing reliable mass production of such laser diodes.
In principle, this invention improves the subject ridge waveguide laser diodes by shaping the ridge waveguide in a particular way. The focal point is the special arrangement of the waveguide broadening or flaring towards the front and/or the back facet. To improve the light output power, the ridge waveguide section is just slightly widened towards the front and/or the back facet, preferably to the same aperture. This reduces the series resistance compared to a standard narrow stripe diode laser if the effective contact area has been increased as compared to the latter. In addition, the lateral gain regime is increased where the power density increases. Due to the lateral induced gain region towards the front facet, the beam can be further amplified, whereas in standard ridge waveguide laser diodes the amplification is locally saturated at lower power levels. The effect of spatial hole burning is thus reduced as compared to a standard narrow stripe waveguide.
Essentially, the novel waveguide design can be expressed as xe2x80x9cLongitudinal Index Managementxe2x80x9d (LIM). LIM provides for the following:
1. the waveguide widening is small compared to common flared laser structures, usually below 10 xcexcm;
2. front and back end-sections are ending in a locally straight waveguide geometry enabling an essentially standard, xe2x80x9cnarrow-stripexe2x80x9d manufacturing process; and
3. preferably front and back end-sections have the same aperture width or cross section.
To realize a flared or tapered ridge waveguide design with the above features, in particular to realize it using an essentially standard manufacturing process, a novel lithographic pattern was also designed. This will be explained further below.
In brief, the invention concerns a semiconductor laser diode with an active region including a ridge waveguide having front and back facets. This ridge waveguide comprises at least three distinct segments along its longitudinal extension:
a center segment extending with a substantially constant first cross section,
a slightly tapered segment widening towards at least one of said facets, and
an end segment close to the facet with a substantially constant second cross section, i.e. essentially with the cross section or aperture of the facet.
It may thus be called a xe2x80x9cstraight-flared-straightxe2x80x9d waveguide design.
Preferably, the ridge waveguide has two tapered segments extending along the ridge waveguide, a first one widening towards the front facet and a second one widening towards the back facet of the waveguide, and two end segments, one associated with each facet. Again, the two end segments have preferably the same cross section as the associated apertures. Thus, it may be called a xe2x80x9cstraight-flared-straight-flared-straightxe2x80x9d waveguide design.
Usually, the two tapered or flared segments have different lengths, in particular the first tapered segment being longer than the second tapered segment and they will be preferably of identical cross section. The two end segments may have substantially the same length, but this is not necessarily so.
Also usually, the ridge waveguide will be structured symmetrically with an essentially constant thickness along its whole length, the widening and the different cross sections being effected by a variation of the width of the ridge wave-guide. Preferably, the center segment is of a first width, e.g. 3 xcexcm wide, and the end segment of a twice said width, e.g. 6 xcexcm wide.
In other words, the ridge waveguide has a long center segment with a substantially constant first cross section, at least one flared segment widening towards one of the facets, and at least one short end segment between said tapered segment and said facet, said short end segment having a substantially constant second cross section larger than said first cross section. Preferably, the ridge waveguide has two flared segments extending and widening from the center segment towards the facets in opposite direction, connecting the long center segment continously with said short end segments.
A novel method for manufacturing a high power ridge waveguide laser diode with facets having identical cross sections forming mirror images enables strip-wise manufacturing with subsequent breaking of the laser diodes despite the flared or tapered structure of the ridge waveguide. This was impossible for flared designs until now.
A ridge waveguide laser diode structured according to the present invention exhibits an extreme improvement over prior art laser diodes, especially with regard to its long-term stability and reliability.
This significant improvement is believed to be mainly caused by the broader end segments, enabling the injection of current over an increased contact area, while the beam stability is given by the straight center segment.
A further essential improvement can be achieved by combining the LIM design approach with a diode laser design termed xe2x80x9cunpumped end sectionxe2x80x9d and described in copending U.S. patent application Ser. No. 09/852,994, entitled xe2x80x9cHigh Power Semiconductor Laser Diodexe2x80x9d, which is incorporated herein by reference. The LIMxe2x80x94xe2x80x9cunpumped end sectionxe2x80x9d combination further enhances the reliability of the pump laser device. The reason is that the current density, as a possible cause for device degradation, is locally reduced at the sensitive front and back end section of the laser.
Advantageously, the improvement needs just a minor increase in manufacturing complexity so that conventional manufacturing equipment may be used and usual manufacturing processes may be applied. Also, the laser diode itself has the same dimensions as before, thus avoiding any packaging changes or problems.